Most minerals occur in nature as mixtures with other minerals as well as valueless materials commonly referred to as gangue. To obtain mineral materials, from which metals can be recovered, it is necessary to separate the mineral materials of the ore material from each other as well as from the gangue. This separation becomes particularly difficult if the ore material is complex as to the mineral structure and the mineral particles are intergrown. Mineral separations are characterized conventionally in terms of "recovery" and "grade" or "product grade". Recovery is the quantity of metal contained in any particular separation product or concentrate expressed as a percentage, often a molar percentage, of that metal contained in the feed, and grade or product grade is the content of a particular mineral or metal in that separation product expressed in terms of the total mass of that product. The effectiveness of a separation is determined by both recovery and grade which must properly be considered together, since the recovery is often inversely proportional to the grade.
The ores which comprise the primary sources of lead and zinc contain these elements in the form of metal sulfides, particularly as galena (PbS) and sphalerite (ZnS). These minerals often occur in an ore in varying proportions and are typically found in association with copper sulfides such as chalcopyrite (CuFeS.sub.2) and pyrite (FeS.sub.2). The conventional method of separation these minerals is by flotation, in particular froth flotation. The ore material is ground, usually by wet grinding, to liberate particles of mineral materials from the gangue materials. The mineral particles are then conventionally conditioned by treatment with collectors, i.e., additives optionally employed with an activator, which are designed to make the desired mineral material particles more hydrophobic, or depressants, i.e., additives designed to make the gangue or other minerals more hydrophilic. The minerals are suspended in an aqueous liquid termed "pulp" and dispersed air is then introduced into the mineral pulp in a stirred tank. The hydrophobic particles become attached to the air bubbles and are carried upwards to be collected in the froth which overflows the tank into a collector. The hydrophilic materials termed "tailings" leave the tank at a location away from the froth discharge and are collected for further processing or are discarded.
For an ore containing lead and zinc as well as copper and iron (as pyrite), a typical sequence is copper flotation, lead flotation, zinc flotation and finally pyrite flotation. Although only a portion of this overall sequence is typically applied to any given ore, there are established separate flotation lines for each mineral and the process is termed differential flotation. Often however, particularly high degrees of separation are difficult to obtain by flotation and mixtures of minerals are floated together in bulk flotation. This bulk flotation is particularly useful when the ore is complex and the minerals are intergrown. The product of a bulk flotation, a primary flotation concentrate, must be further processed, often by further flotation, after cleaning operations to improve the mineral grade by rejection of materials undesirably included within the flotation froth by, for example, mechanical entrainment or intergrowths. In this latter case, regrinding of particles is often required prior to cleaning. The tailings of such cleaner flotation cells are generally recycled to some earlier point in the overall process if the metal content of the tailings is such that the tailings cannot be discarded.
The separation of minerals by flotation is not entirely satisfactory. If the minerals are intricately intergrown, very fine grinding is required to liberate the mineral particles and separation of the resulting fine particles becomes difficult because of similar surface properties. As a result, a number of regrinding and reprocessing steps are required to effect the desired separation. In certain situations, the presence of a third mineral causes a desired separation of two minerals to become more difficult. The presence of cuprous sulfide, for example, may activate any pyrite present and lead to difficulty in separating lead and zinc sulfides from that pyrite.
Alternatives to flotation have been proposed for the separation of ores including liquid-liquid extraction and agglomeration. In the case of complex lead and zinc ores, no acceptable extractive separation has been achieved. While bulk separation is possible, the results obtained are not generally better than those obtained through flotation.
Agglomeration methods involve pretreatment of the minerals by methods similar to the pretreatment employed in flotation processes. The ore is ground and slurried in a stirred tank to establish density differences. Various reagents including depressants, activators and collectors are used to condition the particles as reviewed by Bulatovic et al, "Complex Sulfides," proceedings of a Symposium by AIME, San Diego, Calif., 1985. Reagents used for spherical agglomeration are not necessarily the preferred reagents of a flotation process. The ore particles rendered hydrophobic are conventionally agglomerated with a hydrocarbon liquid under conditions of shear in one or more stages in agitated tanks. The various stages often provide for initial agglomeration and also for agglomerate growth. The agglomerates are then separated by conventional mechanical methods such as screening, hydroclassification, flotation or other physical separation procedures.
Spherical agglomeration of copper-lead-zinc-containing mixtures has been evaluated by House et al, Min. Eng., 2 (2), pp. 171-184 (1989). However, the materials separated were artificial mixtures of chalcopyrite, sphalerite, pyrite and sand (quartz). Separation of such mixtures of these individual materials by agglomeration methods gave good results, but no evaluation of the method on complex, intergrown ores were disclosed. It was suggested, however, that agglomeration processes could be competitive with froth flotation for rough-ground mineral ores if further regrinding and agglomeration stages were used.
Spherical agglomeration separation does not, however, appear to be effective for intergrown particles in which one of the components is a relatively more hydrophobic mineral of relatively coarse particle size. Recovery is efficient only for any liberated material present. The coarse material could be reground, however, for further separation. It would be of advantage to have a simplified processing scheme for the separation of complex, intergrown ore material containing lead and zinc minerals which scheme reduces the number of steps including recycle steps required for separation of the minerals.